I have a Golden Age Project Pre 73 jr pre amp, a Golden Age Project Ribbon microphone R1 MK II and a Martin guitar, quite great quality gear, but when I record through my Roland Duo Capture EX into my iPad Air 2 the sound isn’t very good. I have no acoustic absorber but puts a blanket behind the mic.
Can the problem be that my interface doesn’t do justice to my gear?
Will there be a big difference if I buy a more expensive interface, like someting in the range 300-400 $?

More expensive interfaces (in most cases) cost a lot more because the components inside are of much higher quality.
You probably won't get a huge boost (IMO) in audio quality from your Roland to another interface in the $300-400 range. If you want better quality preamps that you can 'hear', you should be looking at interfaces from RME, Apogee, Antelope, Midas (not Behringer), Lynx, Prism and other similar companies. There are a lot of quality interfaces out there but, they'll cost you.

Here's my two cents. I've had a golden age pre like yours and was using an Audio Technica at4033a mic. With a custom made acoustic guitar with similar voicing as a Martin D28. My interface at the time was a Digidesign digi002r with Pro Tools and a Focusrite scarlett 18i6 (hooked up to Ipad and auria). My room was 10 x12 with acoustic tile ceiling and dry wall on three sides and one brick wall. I did have several acoustic panels up for a dead sound. Never could get an acoustic guitar sound that was any good. changes mics to a shure sm81 and was the best choice for the the job. but still not very good recordings. It wasn't until I went to a friends house (guest cabin) one long room about 20 x 40 with a angled ceiling. Night and day difference in recording. no longer did I have a boomy guitar, no need for any bass roll off. It opened my eyes up to the fact that the room has more to do with recording than the equipment. Any modern Interface with a SM81 or descent condenser mic will record a great acoustic guitar sound (as long as your guitar sound great) if your in a nice sounding room. I've tried recording in all kinds of rooms after I was inlightened with the knowledge and now its super easy to know if a room is adequate for recording in. I would say not the equipment but the room (a ribbon mic will be dark so I would suggest a bright room).

There are plentyof tutorials on youtube on how to record acoustic guitar. There are many ways to do it using many different mic,placement,preamp,ect. My advice is to experiment. Try different mics and placement. New strings really help. It really depents on the sound your looking for. There is no one way to record anything but there are some go to methods that get you a good recording without too much fuss. Rooms play a part but they are not what makes a good recording. In fact if your looking for variety in your mixes or tracks using the same mics,guitars and rooms can turn into the same sound each and everytime you record and this can be a drawback. Unless thats what you need is the same sound every time you record your guitars. You can also try blending a DI signal of the guitar with the mic’d tracks to achieve a different tone. Remember you can really save a track in the mixing stage. I do this all the time as some of the guitar tracks I receive to mix a way less than perfect but with some good mixing I can bring them to life. Good luck and enjoy the most powerful mixing daw on iOS. Cheers.

I have a Golden Age Project Pre 73 jr pre amp, a Golden Age Project Ribbon microphone R1 MK II and a Martin guitar, quite great quality gear, but when I record through my Roland Duo Capture EX into my iPad Air 2 the sound isn’t very good. I have no acoustic absorber but puts a blanket behind the mic.
Can the problem be that my interface doesn’t do justice to my gear?
Will there be a big difference if I buy a more expensive interface, like someting in the range 300-400 $?